The present invention relates to an alarm system utilizing a bidirectional wired television system, such as, a bidirectional community antenna television system or CATV system.
The known systems for providing an alarm service to a large number of subscribers from a remotely located centralized station or central station include the polling systems of the type as disclosed in U.S. Pat. No. 3,765,016 in which a plurality of subscribers are sequentially interrogated from a central station to answer if there has occurred any abnormal condition and the contention systems of the type as disclosed in U.S. Pat. No. 3,996,578 in which those subscribers detecting the occurrence of an abnormal condition in their monitoring areas send alarm signals to a central station.
The network of coaxial cables for the CATV system is generally so arranged that the coaxial cables are branched off to the subscribers. U.S. Pat. No. 3,765,016 discloses a system in which a central station is connected to a transmission loop including a plurality of series connected subscribers and a normally closed line relay is connected in series with each of the subscribers in the loop, and consequently this system cannot be carried out by utilizing the above-mentioned branched CATV network of coaxial cables as such. Another disadvantage of this prior art system is that since the interrogating signal used for polling is in the form of a pulse code signal and since the subscribers are connected in series with the transmission line, the cycle time required for polling is long and consequently if the number of subscribers is large, the time interval from an interrogating signal until the subscriber receives the next interrogating signal is increased, thus making it impossible to send an early alarm.
On the other hand, with the contention type alarm system disclosed in U.S. Pat. No. 3,996,578, different frequencies are assigned to the respective subscribers and another different frequencies are assigned to the respective group units each including a plurality of subscribers whereby any subscriber sending an alarm is discriminated in accordance with the values of the frequencies, and thus it is necessary to use a large number of frequency discriminating devices such as demodulators, tuners or band pass filters. Another disadvantage is that where alarm signals include signals which indicate the types of abnormal conditions, such as, fire, burglary and gas leakage and the central station is required to discriminate the types, the assignment of frequencies becomes increasingly difficult with an increase in the number of subscribers and consequently there exists inevitably an upper limit to the number of subscribers in order that the subscribers as well as the types of abnormal conditions may be effectively discriminated within a limited frequency band.